Dynamical Mechanism behind the Southern Hemisphere Westerly Intensification during the Last Glacial Maximum: A Linkage between Sea-ice and Polar Front Jet

Abstract. The Southern Hemisphere westerlies (SHW) play a pivotal role in modulating the global carbon cycle and climate feedback. However, their behavior during the Last Glacial Maximum (LGM) is debated owing to discrepancies between paleoclimate models and proxy records. While tropical upper-tropospheric cooling and Antarctic surface cooling exert opposing influences on the SHW, the detailed dynamical mechanisms through which Antarctic sea-ice expansion modulates large-scale atmospheric circulation are poorly understood. In this study, we investigated the dynamical mechanisms of austral winter SHW change under altered orbital and surface conditions with a series of climate model simulations. By conducting sensitivity experiments with varying Antarctic sea-ice concentrations, we isolated the thermodynamic effect of sea ice from the tropical cooling signal. Our results demonstrate that sea-ice-induced surface cooling drives the poleward intensification of the SHW through two distinct mechanisms. First, strong surface cooling steepens the meridional temperature gradient near the sea-ice edge, thereby directly maintaining the SHW intensity through thermal wind balance. Second, the enhanced baroclinicity amplifies eddy heat fluxes and storm track activity. The resulting increase in storm track activity drives a downward transfer of upper-tropospheric westerly momentum, reinforcing the surface westerlies. Through these mechanisms, the sea-ice-driven cooling outweighed the opposing equatorward influence of tropical cooling. This study provides a dynamical framework for understanding how sea-ice thermodynamic forcing drives large-scale circulation changes in the context of LGM climate conditions.